The present invention relates generally to a device for in situ detecting of axial motion of a generally cylindrical member and, more particularly, to such a device for detecting such axial motion in situations in which both ends of the cylinder member are unavailable for the installation of a prewound coil.
Numerous prior art systems have been developed for the evaluation of valve and valve operator systems, particularly systems which are motor or otherwise power driven and are operated from a remote location Such valves are common in the utility industry and are sometimes used for the protection of power equipment, as well as for the protection of the general public from the release of hazardous materials, either directly or indirectly. One such system is disclosed in U.S. Pat. No. 4,542,649, the disclosure of which is incorporated herein by reference.
The background section of the aforementioned patent describes in detail the problems of the prior art and the need for an improved method for monitoring and evaluating the condition and performance of a valve and valve operator combination, as well as the need for diagnosing particular operational problems. Typically, such prior art systems correlate valve stem load or forces to the displacement of a spring pack by backseating the valve stem against a calibrated load cell while recording the displacement of the spring pack resulting from spring pack compression by utilizing a displacement sensor. The resulting calibration is then used to infer stem loads, seating and backseating forces as a result of spring pack displacement alone.
The problem with the above-described and other prior art systems is that the spring packs typically utilized in conjunction with valve operators have substantial initial compression, requiring that the force on the valve stem build up to a significant predetermined level before the spring pack begins to compress beyond the initial compression and to displace. This feature of the spring pack leaves a large dead zone within which no valve stem force measurement can be made.
In addition, there may be spacing or clearance between the spring pack and the spring pack cavity which allows the spring pack to displace the amount of the clearance in response to a minimum or no corresponding force being imposed on the valve stem. Such clearance adds additional error to such prior art systems. Even if the spring pack was tightly placed within the spring pack cavity with no such clearance, and even if there was no initial compression on the spring pack, the compression of the spring pack tends to be non-linear with respect to the forces encountered by the valve stem. Therefore, spring pack displacement could not provide an accurate indication of valve stem forces over the entire range of spring pack displacement.
Furthermore, it is not uncommon for grease or other lubricants to build up within the spring pack, possibly filling the entire spring pack cavity. Such a buildup of grease or other lubricants can greatly limit spring pack compression, even under the application of very large valve stem forces and, therefore, spring pack displacement would not accurately reflect valve stem forces. Finally, friction forces on the gears within the valve operator could also cause the spring pack to compress, even though no forces are actually being placed upon the valve stem. In short, because of the problems of such prior art systems, an improved means for providing a more accurate and precise measure of valve stem forces is needed. It is also necessary to provide such a measurement method utilizing indirect measurement techniques since the valve stem itself is generally not accessible over the entire valve stroke, therefore making direct measurement difficult and infeasible.
The present invention overcomes the problems of the prior art systems by measuring valve stem forces utilizing sensor means secured to he valve yoke. The invention operates on the premise that the forces on the valve stem are opposed by equal and opposite forces in the valve yoke and that the measurement of yoke deflection or strain results in a proportional indicator of the valve stem forces. Unlike the methods employed by the prior art, such a yoke strain measurement provides a linear measure of valve stem forces from zero force upwardly which is unaffected by all of the above-discussed spring pack anomalies.
The present invention also provides a system for evaluating the condition and performance of the valve and the valve operator, and for identifying malfunctions or other problems in either the valve or valve operator, utilizing the valve stem force measurement (obtained from valve yoke strain measurements) in conjunction with other measurements, including valve stem motion, motor current, spring pack motion and torque and limit switch indications. In addition to diagnosing current valve and valve operator problems, the present invention provides a method for trending changes in overall valve and valve operator conditions in order to predict future valve or valve operator problems.